1. Field of the Invention
The invention is generally related to fixed offshore platform structures and more particularly to configurations of these structures that result when only the smallest one or two well casings are extended back from the seafloor to the platform deck and when these casings are supported in tension.
2. General Background
Each offshore oil and gas well begins with a pipe called a "conductor" that penetrates the ocean floor for several hundred feet. Traditionally, conductors have constant diameters along their lengths, usually from twenty-four to thirty inches. The conductor often extends from the sea floor back to the platform's deck level. However, when the well is drilled from a floating vessel, the conductor extends only five to ten feet above the sea floor. The primary function of the conductor is to provide a support foundation for the weight of the well components during drilling. For conductors extended back to the deck level, it has the additional functions of supporting and protecting the well casings in the water and air zones between the rig and the sea floor. The rest of the well consists of a hole lined with a series of concentric steel pipes called "casings" where each casing is smaller in diameter and extends deeper below the sea floor than the preceding one until the last and smallest diameter casing reaches from the top of the well to the oil and gas bearing formation. A typical well might have, in addition to the conductor, casings with diameters of twenty inches, thirteen inches, nine and five-eighth inches, and possibly a seven and five-eighth inch casing.
The well is started by drilling a hole that is smaller than the inside diameter of the conductor but larger than the diameter of the first casing. When this first hole has reached its planned depth, usually in the range of two thousand feet below the sea floor, the first casing is assembled from sections and lowered into the hole until it nearly reaches the bottom. The casing is suspended from the top of the conductor and grouted to the soil up to the bottom of the conductor and then to the inside of the conductor up to at least the sea floor. Each successive casing is installed similarly with each one grouted to the soil and to the previous casing up to at least the sea floor. To complete the well, production tubing is run, the production zone containing the oil and gas is isolated, and the formation perforated through the casing to allow the hydrocarbons to flow into the tubing and up to the platform.
The portion of a conductor that extends between the mudline and the platform deck must be able to resist the horizontal loads applied by the offshore environment of waves, current, and wind. Since the distance from the mudline to the deck is usually significantly greater than the conductor can span as an unsupported, side loaded column, the conductors are supported at appropriate levels in the fixed platform by passing through sleeves which are framed into the structure. The sleeves deliver the horizontal loads imposed on the conductors to the platform's major framing elements. These loads are significant in the design of an offshore fixed platform.
Offshore wells can be drilled either by a rig supported from the platform itself, called a "platform rig", or by a rig that does not use the platform structure for support. The platform rig is composed of multiple modules that are lifted onto the platform by either a crane on the platform or by a large floating derrick and then hooked together. This type of drilling rig applies a large loading to the platform due to its weight and wind area. These loadings contribute significantly to the cost of the platform. This approach is generally used where a sufficiently large group of wells, usually nine or more, will be supported by one platform.
To drill an offshore well without supporting the rig with the platform, either a bottom founded, self-elevating rig called a "jack-up", or a floating rig can be used. With a few exceptions, the jack-up is generally designed for water depths up to three hundred feet while the floating rigs can drill in three hundred feet to over ten thousand feet of water. For both types of drilling, the structure still supports the wells as described above and additionally described below, but, by not supporting the drilling rig, the platform structure can be lighter and more economical.
As originally practiced, the jack-up rig positioned itself alongside a previously installed platform, located its derrick over a conductor, and drilled each well in succession using the same technology as the platform rig. Later, well drilling techniques and casing hardware called "mudline suspension and tieback systems" were developed which allowed the jack-up rig to drill the well prior to the installation of the platform and then temporarily disconnect the sections of conductor and casing between the sea floor and the surface and leave the site. The technique has the distinct advantage of allowing the platform fabrication and drilling operations to proceed simultaneously. By adding a subsea wellhead to the mudline suspension and tieback systems, floating rigs can also drill the wells in advance and then disconnect and leave before the platform is installed.
Once the platform is installed, the tieback hardware allows the well casings to be reconnected at the sea floor and extended back to the platform deck. This is most economically done with a platform type rig called a "workover/tieback" rig. These rigs are smaller than a full drilling rig since they only have to support the casing down to the mudline. In these cases, the conductor itself may or may not be extended back to the deck. However, it is the practice to extend at least the first casing plus one, if not all, of the smaller diameter casings back to the deck. This means that the platform structure still has to support a rig of some intermediate size as well as support the horizontal, environmental loadings imposed on the large diameter casings by the waves, current, and wind.
With the development of fields in waters beyond the economical reach of fixed platforms, floating platform concepts were developed, particularly the Tension Leg Platform(TLP) and the Spar Platform(Spar). A TLP is a semi-submersible vessel held on station by tendons that are tensioned against a foundation at the sea floor and the buoyancy of the vessel at the surface. The SPAR is a floating, vertical cylinder, catenary moored on location. These configurations presented efficient systems of platform support in very deep water but there was no structural system available between the floating platform and the sea floor to provide lateral support for the tied back well casings at intervals along their lengths.
A two-part solution was developed to provide this support. First, each tied back casing is tensioned sufficiently to eliminate the need for intermediate lateral support on both types of floating systems. On the TLP, since the vessel moves laterally a large amount and vertically by a smaller but still significant amount, a constant tension is maintained on each casing with a complex mechanical device called a "tensioner". On the SPAR, this tension is maintained by dedicated buoyancy tanks attached to each casing. Second, for both systems, only the smallest (or perhaps the two smallest) casing string is tied back. Generally, this will be the last one (the nine and five-eighth inch casing) instead of the first one (the twenty inch casing), described earlier for fixed platforms. The smaller casing is lighter and offers a much smaller profile to the wave forces. This in turn requires much less tension to support both the casing's weight and its lateral loadings.
From work on TLPs, Spars, and other floating concepts, appropriate design procedures have been developed for tensioned, tied back casing strings, commonly called "tensioned production risers". Tensioned riser design and construction is well understood by those practiced in the art. In addition, the necessary hardware, including the tensioners and the mudline suspension and tieback systems, is well developed and commercially available.
The tensioned riser concept has most recently been applied to Compliant Tower Platforms which are a type of offshore structure distinct from either fixed platforms or floating platforms such as TLPs and Spars. A compliant tower using the tensioned riser concept has been developed by Smolinski, Morrison, Hute, and Marshall and is described in Paper Number 7450 of the 1994 Offshore Technology Conference(OTC).
The highly specialized nature of the offshore fixed platform industry has presented problems in improving the technology used. It is Well known to offshore platform designers that the well conductors above the seabed contribute a major portion of the total environmental loading on the platform (typically twenty to 30 percent). However, since the platform design engineers have historically executed their designs using specific criteria provided by the drilling and production specialists of the oil companies, the platform designer did not need to know much about drilling technology in order to execute a competent and safe design. The oil companies are further specialized between those who procure the platforms and those who direct the drilling and reservoir development. The natural communication gaps caused by the specialization in the industry has inhibited development of more efficient arrangements.